Radio receivers often include squelch circuitry for preventing output of a received signal having an unacceptable level of noise. A typical squelch circuit may be operated by signal energy in the receiver passband, by noise quieting, or by other techniques. The prior art also describes carrier squelch techniques, in which an audio output is enabled when a carrier signal is detected having an acceptable signal to noise ratio.
A radio receiver generally includes a front-end portion, immediately following the antenna, that operates across a frequency band to process signals associated with a carrier frequency on a particular channel. The frequency band is sufficiently wide to allow for variation of the carrier frequency within a particular range. This ensures that a carrier signal can be detected within certain operating tolerances of the transmitter, the receiver, and the communication environment. A wide frequency band, however, permit other signals to potentially affect the detection of a carrier signal within the frequency band. For example, signals from adjacent channels may be present in the frequency band and thus trigger a carrier squelch detect signal.
Squelch circuit designs may include protection against falsing. Falsing occurs when a carrier squelch detect is triggered by a signal from an interferer or from an adjacent channel that appears to be an on-channel carrier signal. Hysteresis and multiple sampling techniques have been used in the prior art to protect against falsing. It is also common to use a narrow bandpass filter in the radio front end circuitry eliminate all but those frequencies close to the desired carrier frequency. Such filters and additional processing components add to the cost and complexity of prior art receivers.
Direct conversion receivers are known in the art to offer significant advantages in terms of cost and complexity in radio designs. However, direct conversion receivers have reduced protection from frequencies outside the area of interest. Hence, extra protection against falsing must be included for squelch circuits that operate in conjunction with such receivers. Narrow bandpass filters commonly used in heterodyne type receivers are not suitable for direct conversion receivers. Preferably, falsing protection should be provided without using additional components which could increase radio design cost and complexity. Any solution should provide a timely response to a valid carrier signal present within the channel of operation of the radio, while reducing the effects of spurious signals. Therefore, a new approach to carrier squelch detection is needed.